The present invention relates generally to down-the-hole drill (DHD) hammers. In particular, the present invention relates to a drill bit with nonconcentrically arranged or spirally arranged cutting inserts for a down-the-hole drill hammer.
Typical DHDs include a hammer having a piston that is moved cyclically with high pressure gas (e.g., air). The piston generally has two end regions that are exposed to working air volumes (i.e., a return volume and a drive volume) that are filled and exhausted with each cycle of the piston. The return volume pushes the piston away from its impact point on a bit end of the hammer. The drive volume accelerates the piston toward its impact location on the back end of the drill bit. The overall result is a percussive drilling action.
Conventional drill bits used in DHD applications are typically constructed to include cutting inserts that are positioned at a bottom end face of the drill bit. The cutting inserts serve to cut rock or other material upon impact and the drill bit is rotated or indexed a certain amount after each cycle of the piston to reposition the cutting inserts upon a different region of a bore hole.
A typical DHD hammer includes a drill bit having cutting inserts arranged in concentric rings in the distal face or cutting face of the drill bit. In most cases, the rings are separated by at least the diameter of the cutting inserts whereby the concentrically arranged cutting inserts produce concentric grooves or valleys in the rock being drilled as the drill bit rotates and the percussive action penetrates the cutting inserts into the earth. The development of such concentric grooves or valleys increases the energy required for rock breakage. In addition, it is known that rock breakage due to blunt penetration is more efficient when free faces are in close proximity to the Hertzian stress field created during the rock loading process below the point of penetration. For example, it is easier to break a convex rock shape than a concave one. It would therefore be desirable to have drill bits that minimize or eliminate the degree of concentric grooving created by the concentric placement of cutting inserts in conventional drill bits.